Buy the best configuration you can afford and don't look back. The iMac has every expansion port you'll likely ever need and TB3 makes adding super fast storage stupid easy. OWC sells a tiny TB3 box that can house 4 NVME m.2 sticks.
So get an iMac over a Mac Mini for the sleek design and lack of wires, then attach a bunch of stuff to it and put wires everywhere?
I'd actually consider this less of an issue on a Mac Mini because you're already buying a separate device there. but when you're buying an AIO form factor - forcing any expansion to be done on the outside is less forgivable.